Typically tubing hanger installation for either a conventional or horizontal subsea Christmas tree system utilises a riser as a method of lowering the tubing hanger to the wellhead/Christmas tree and as a means of transporting fluids to and from the wellbore. The riser also acts as a means of transporting wireline and coiled tubing from the surface to the desired location. The typical arrangement of installation equipment is as shown in FIGS. 1a–1d, with FIG. 1a showing a “conventional” completion and FIG. 1b a horizontal completion. In FIG. 1a, a BOP 10 is landed on and sealed to a wellhead 12. A marine riser 14 extends from the BOP 10 to a drilling vessel (not shown). The completion landing string comprising a tubing hanger (TH) 16 and associated tubing (not shown), tubing hanger running tool (THRT) 18 and tubing hanger orientation joint (THOJ) 20 is lowered into the marine riser 14 on a dual bore high pressure riser 22. A controls umbilical 24 is secured to the riser 22 and extends from the drilling vessel to the THOJ and THRT. A surface tree 26 is secured to the riser 22 for control of well fluids. The corresponding FIG. 1b arrangement for a horizontal tree 28 comprises a BOP 32 secured to the tree 28, and a landing string comprising a THRT 30 for TH 34, a subsea test tree (SSTT) 36, an emergency disconnect package (EDP) 38, a retainer valve 40, a monobore riser 42 and a controls umbilical 44; all run through a marine riser 46. A surface tree 48 is secured to the monobore riser 42. If required, fluid communication with the tubing annulus may be established via the BOP choke and kill lines 45, 47, or via a separate external connection (not shown).
For wireline operations, a lubricator 50 is attached to either surface tree 26 or 48, as shown in FIG. 1c. Similarly, a tubing injector 52, comprising a tractor unit 54 and stuffing box 56, may be attached to the surface trees 26, 48 for coiled tubing (CT) operations.
The high pressure riser system represents a sigificant proportion of the installation equipment total cost and can, in the case of small projects, significantly affect the profitability of individual wells. Historically the riser systems, which are usually purpose designed pipe-pipe coupling equipment, are regarded as non-reusable and have long lead times to design and produce for each project. In the case of deepwater wells the time to run equipment can significantly affect the overall installed cost of a well. Furthermore, although some investigations into riserless drilling of the well have been carried out, completion equipment currently in use requires a high pressure riser for instaltion of the tubing hanger. This negates some of the cost savings available from riserless drilling. Therefore elimination of the riser system will significantly reduce project costs and lead times.
For deep water applications, a dynamically positioned installation vessel is typically used and emergencies concerning vessel station keeping are more likely to arise. This is of partcular concern during extended well flow testing. It is desirable to improve speed and reliability of emergency disconnection of the riser system from the BOP.
U.S. Pat. No. 5,941,310 (Cunningham) discloses a monobore completion/intervention riser system, providing a conduit for communicating fluids and wireline tools between a surface vessel and a subsea well. A ram spool is provided, engageable by BOP pipe rams, to establish fluid communication between an annulus bore and a choke and kill conduit in the BOP.
U.S. Pat. No. 5,002,130 (Laky) and U.S. Pat. No. 4,825,953 (Wong) disclose open water, subsea CT injectors and wireline lubricators, but do not suggest the use of such equipment in subsea completion operations, which normally utilise a BOP and marine riser attached to the wellhead.